Well drilling methods such as rotary drilling method and riserless drilling method as well as hydraulic fracturing method have now been widely employed for extracting underground resources.
The rotary drilling method consists of forming the well by drilling while refluxing the mud and forming a filter cake called mud wall on the wall surfaces of the well using a finishing fluid blended with a water loss-preventing agent. The cake maintains the chute walls stable, prevents the chute walls from collapsing and reduces friction to the fluid flowing through the well.
The hydraulic fracturing method consists of pressurizing the fluid filled in the well to form cracks (fractures) in the vicinities of the well to thereby improve permeability in the vicinities of the well (for easy flow of the fluid) in an attempt to increase the effective sectional area through which the resources such as oils and gases flow into the well and, therefore, in order to improve productivity of the well.
Here, as the water loss-preventing agent that is added to the finishing fluid, there are chiefly used calcium carbonate or various kinds of salts in a granular form. However, use of the water loss-preventing agent brings about such problems that it becomes necessary to conduct a treatment with acid to remove it, or the grains enter into the stratum and, specifically, into cracks in the stratum to block the flow of gases. That is, the water loss-preventing agent stays clogged in the stratum from where the resources are to be extracted hindering the production.
Further, the fluids used in the hydraulic fracturing method can be grouped into a fracturing fluid and a filler, the fracturing fluid being used so as to permeate into the vicinities of the well under the application of a high pressure while the filler being used in order to block the flow passage in the well. As the fluid for use in the hydraulic fracturing method, there has heretofore been used a viscous fluid like jelly gasoline. In recent years, however, as the shale gas or the like gas has now been extracted from the shale layer that exists in relatively shallow places and by taking the effects on the environment into consideration, it is becoming a common practice to use an aqueous dispersion solution obtained by dissolving or dispersing a polymer in water. A known example of the polymer is polylactic acid (see a patent document 1).
That is, the polylactic acid is a substance that exhibits hydrolysable capability and biodegradable capability, and, even if it remains under the ground, is decomposed by water or enzyme in the ground and does not adversely affect the environment. Further, the water that is used as a dispersant, too, can be considered to be far from affecting the environment as compared to gasoline or the like.
The well is filled with the aqueous dispersion solution in which the polylactic acid has been dispersed as the fracturing fluid and is pressurized so that the polylactic acid permeates to the vicinities of the well. Here, the polylactic acid undergoes the hydrolysis and loses the form of the resin. Therefore, spaces (or cracks) form in the portions through where the polylactic acid had been permeated accounting for an increase in the space of the well into which the resources can flow.
Further, the polylactic acid also works as a water loss-preventing agent. That is, by forming the filter cake in the well by using the finishing fluid that is blended with the polylactic acid as the water loss-preventing agent, it is made possible to suppress the water contained as a dispersion medium in various fluids used in the subsequent steps of extraction from permeating into the ground too much. Therefore, the polylactic acid offers an advantage of minimizing a change in the environment in the stratum. Besides, no treatment with acid is necessary since it decomposes in the ground.
In addition, the lactic acid which is decomposed from the polylactic acid is an organic acid. As the polylactic acid decomposes, the lactic acid is released. The lactic acid corrodes the shale layer and accelerates the shale layer to become porous.
However, though the polylactic acid undergoes the hydrolysis relatively quickly at temperatures of not lower than 100° C., its rate of hydrolysis is small at temperatures of lower than 100° C. If used for extracting, for example, the shale gas from under the ground where the temperature is low, therefore, the efficiency of extraction becomes poor and improvements are desired.
On the other hand, a proposal has been made to use a polyglycolic acid in place of the polylactic acid (see a patent document 2).
The polyglycolic acid, too, has been known to be useful as a biodegradable resin. Besides, its hydrolysable capability is higher than that of the polylactic acid; i.e., the rate of its hydrolysis at a temperature of, for example, about 80° C. is considerably larger than that of the polylactic acid and it can be effectively used to substitute for the polylactic acid. At temperatures of not higher than, specifically, 80° C. and, more specifically, not higher than 60° C., however, the rate of hydrolysis of the polyglycolic acid becomes very small. Besides the polyglycolic acid is considerably expensive as compared to the polylactic acid.